1. Field of the Invention
The present invention relates to vehicle communications networks and more particularly relates to providing a system and method for testing such networks for electromagnetic. interference (EMI) susceptibility
2. Description of the Prior Art
Contemporary designs for the control and management of vehicle components increasingly rely on methods derived from computer networking. Digital data is exchanged between component controllers over a common physical layer, such as a twisted shielded pair of wires. Intelligible communication between two or more device controllers among a greater plurality of devices, all occurring over the common physical layer, depends upon the communicating devices being able to discriminate among messages they receive and respond to those messages directed to them. Such methods are well known in the art and are part of the standards which the Society of Automotive Engineers (SAE) has published and continues to publish as part of the SAE J1939 protocol.
The J1939 protocol provides an open protocol and a definition of the performance requirements of the medium of the physical layer, but also allows for development of proprietary protocols. The SAE J1939 protocol is a specialized application of a controlled area network (CAN) and may be readily implemented utilizing commercial integrated circuits such as the C167 Integrated Circuit from Stemens of Germany.
The CAN protocol is an ISO standard (ISO 11898) for serial data communication, particularly aimed at automotive applications. The CAN standard includes a physical layer (including the data bus) and a data-link layer, which define a few different message types, arbitration rules for bus access and methods for fault detection and fault confinement. The physical layer uses differential transmission on a twisted pair wire bus. A non-destructive bitwise arbitration is used to control access to the bus. Messages are small, at most eight bytes, and are protected by checksum error detection. Each message carries a numeric value which controls its priority on the bus and typically also serves as an identification of the contents of the message. CAN offers an error handling scheme that results in retransmission of messages when they are not properly received. CAN also provides means for removing faulty nodes from the bus. CAN further adds the capability of supporting what are termed xe2x80x9chigher layer protocolsxe2x80x9d for standardizing startup procedures including bit rate setting, distributing addresses among participating nodes or kinds of messages, determining the layout of the messages and routines for error handling on the system level.
Digital data communications over serial data paths are an effective technique for reducing the number of dedicated communication paths between the numerous switches, sensors, devices and gauges installed on the vehicles. Multiplexing the signals to and from local controllers and switches promises greater physical simplicity through displacing much of the vehicle wiring harness, reducing manufacturing costs, facilitating vehicle electrical load management, and enhancing system reliability. However, such systems are not immune to electromagnetic interference (xe2x80x9cEMIxe2x80x9d). The physical layer of the communication network is, in effect, an antenna, which converts electromagnetic radiation into electrical signals on the physical layer. These signals can combine with data pulses in ways that change the values of the data pulses. Changing a single data point (bit) in a data package makes the data package useless to the intended destination, and may even prevent the destination from decoding the signal at all.
The design of vehicles, and of particular interest here, trucks, requires consideration of the EMI susceptibility of the vehicle""s communication system. Testing of such systems, as part of the design and development of trucks, is carried out in an intense EMI environment with the truck mounted on rollers so that EMI susceptibility may be determined during operation of the vehicle. The testing environment provides a roller bed for the truck, with the roller bed possibly disposed on a turntable. An EMI source is aimed at the truck. In effect, an objective of the testing is to make the vehicle""s communication network as inefficient an antenna as possible. The specifications for the testing are set forth in the SAE Standard J551, parts 11, 12 and 13.
During testing a datalink or telemetry probe is connected into a data bus diagnostic port on the truck, and may be positioned in the truck cab under the steering column. The datalink runs from the port to a location away from the truck, where it is connected to monitoring equipment. These data links have typically been constructed of a twisted pair cable. The external data link is severely limited in length, in large part because the cable acts as an extension of the antenna formed by the physical layer of the truck""s network. Because the external cable changes the dimension and length of the antenna, it also changes the susceptibility of the combined system. This can result in additional errors to messages transmitted on the truck""s CAN network, or, under some circumstances, it can result in fewer errors than would otherwise occur. In either case the test results are suspect.
Because the external link acts as an antenna itself, or as an extension of the antenna formed by the physical layer of the truck""s data network, the external data link distorts the conditions of the test. More accurate direct measurements can be obtained if the affects of the external data link are minimized.
It is an object of the invention to provide a reliable controller area network communication link in an EMI intensive environment.
It is another object of the invention to provide a test data link into an electrical data communications system which minimizes changes to the EMI susceptibility of a vehicle communications system.
It is a still further object of the invention to provide a CAN based repeater able to simulate a portion of a CAN network.
It is yet another object of the invention to provide extension of the CAN network to a remote location during EMI susceptibility testing.
It is still another object of the invention to provide qualification of a optical interconnection to an electrical controller area network.
According to the invention there is provided a data transmission system for external use with a controller area network installed on a motor vehicle undergoing high intensity electromagnetic interference susceptibility testing. The data transmission system includes a fiber optic cable extending from the immediate vicinity of the truck to a site relatively remote to the truck and out of the effective area of the artificially generated electromagnetic interference. An optical/electrical coupler in close proximity to the motor vehicle is connected to the vehicle controller area network to convert messages occurring on the network to optical signals. The connection is provided by an electrical cable constructed of a twisted pair of wires. The optical/electrical coupler is also connected to an end of the fiber optic cable in the vicinity of the truck. A remote data processing device is attached to the remote end of the fiber optic cable by an interface card installed in the remote data processing device. The interface card converts optical signals to controller area network compatible electrical signals for evaluation.
Additional effects, features and advantages will be apparent in the written description that follows.